Always update target version prefix

When UpdateAttempter::Update is called, the cached target version
prefix in its |omaha_request_params_| should always be updated with
the one passed in.

Also fix the target version LOG lines in kiosk app pin policy code.

BUG=chromium:695133
TEST=UpdateAttempterTest.TargetVersionPrefixSetAndReset

Reviewed-on: https://chromium-review.googlesource.com/446657
Commit-Ready: Xiyuan Xia <xiyuan@chromium.org>
Tested-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Alex Deymo <deymo@chromium.org>
Reviewed-by: Ben Chan <benchan@chromium.org>

(cherry picked from commit 4d34c18e5ce29d138c7946cc817dee8b3fce7cf0)

Change-Id: I712b169505f4566aac3a0b2c9746dfcb9466ef8d
diff --git a/update_attempter.h b/update_attempter.h
index f6547fe..c88e7ad 100644
--- a/update_attempter.h
+++ b/update_attempter.h
@@ -256,6 +256,7 @@
   FRIEND_TEST(UpdateAttempterTest, UpdateTest);
   FRIEND_TEST(UpdateAttempterTest, ReportDailyMetrics);
   FRIEND_TEST(UpdateAttempterTest, BootTimeInUpdateMarkerFile);
+  FRIEND_TEST(UpdateAttempterTest, TargetVersionPrefixSetAndReset);
 
   // CertificateChecker::Observer method.
   // Report metrics about the certificate being checked.